Obama and Zardari "had a brief conversation," and were joined later by Afghan President Hamid Karzai to take a joint picture, National Security Council spokesman Tommy Vietor said.

NATO invited Zardari to the summit amid signs that Pakistan could soon reopen ground supply lines into Afghanistan. Pakistan shut the routes after a November incident in which a NATO strike on its soil killed 24 of Pakistani soldiers.

American and NATO officials say the closure of the routes has not hurt the alliance's war on the Taliban, but that they would be crucial to the plan to withdraw the International Security Assistance Force's 130,000 troops by the end of 2014.

"So far, the closure of the transit routes have not had a major impact on our operations in Afghanistan," NATO Secretary-General Anders Fogh Rasmussen told reporters.

"But it goes without saying that it will be quite a logistical challenge to draw down the number of troops in the coming months and years. So we need a number of transit routes, and obviously the transit routes through Pakistan are of great importance, and I would expect a reopening of the transit routes in the very near future," Rasmussen said.